nvme: read the subsystem NQN from Identify Controller

NVMe 1.2.1 or later requires controllers to provide a subsystem NQN in the
Identify controller data structures.  Use this NQN for the subsysnqn
sysfs attribute by storing it in the nvme_ctrl structure after verifying
it.  For older controllers we generate a "fake" NQN per non-normative
text in the NVMe 1.3 spec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>